Abstract This is an R21 proposal to investigate Phospholipase D4 (Pld4) a novel gene of unknown function with homology to classical PLD enzymes. Pld4 has been linked to inflammatory and autoimmune diseases. We generated a conditional knockout allele of Pld4, Pld4 monoclonal antibodies, and Pld4-mutated cell lines using CRISPR to study Pld4 expression and function. Pld4-/- plasmacytoid dendritic cells were found to have defects in type I interferon and other cytokine responses to endocytosed nucleic acid ligands. The present proposal explores several hypotheses related to how Pld4 might function in pDCs and other leukocytes to facilitate these responses. The immediate goal is to understand how Pld4 functions in pDCs and other leukocytes. Long-term goals are to understand all of the roles of Pld4 in health and disease and to explore whether Pld4 might be a good target for therapeutic intervention, particularly in autoimmune disease or vaccinology.